1. Field of the Invention
Time-division multiplex synchronous digital transmission signals generally use a frame structure marked by alignment words enabling location of the data at the receiving end and transmission codes which include block codes introducing some degree of redundancy to facilitate timing recovery and transmission error detection and adapting the form of the signals to the characteristics of the transmission channels.
2. Description of the prior art
The use of a block code requires division of the digital bit stream into successive blocks of the same size (in the order of ten data bits) while the use of frames leads to division of the digital bit stream into consecutive sequences of exactly the same size (this size is relatively large and can be as much as several hundred data bits or more), each marked by a special configuration of bits in the same form as the data, called the frame alignment word. The frame alignment words are either localized or distributed. The division of the digital bit stream into blocks is compatible with its division into frames in the sense in that the length of a frame is a multiple of that of a block, a frame comprising several dozen consecutive blocks, and in the sense that the frame alignment words all have the same position relative to the block boundaries.
Frame synchronization consists in synchronizing a frame timebase generating a frame timing signal to the received frame alignment words. One common way of achieving this consists in systematically searching the received digital bit stream for a configuration identical to that of a frame alignment word and regarding this as a true frame alignment word provided that it repeats in the received digital bit stream with the same period as a frame. A disadvantage of this method is that whenever frame synchronization is lost a relatively slow resynchronization process results if there is a high probability that the frame alignment words will be imitated by the data.
To combat this slowness of the resynchronization process without using excessively long synchronization words it is known, especially from U.S. Pat. No. 4,316,284, to introduce into each frame a cyclic code word which changes from frame to frame in a known way to enable a posteriori verification of frame synchronization. The configuration of this cyclic code word is variable from one frame to another and is very unlikely to be imitated by the data, and this makes it possible to detect incorrect synchronization very quickly, virtually within the duration of one frame, and to abandon the process without waiting for a new attempt to synchronize on the next alignment word configuration detected in the received digital bit stream. This avoids synchronizing on imitation alignment words in several consecutive frames.
In the case of a synchronous digital bit stream divided into blocks by a block code and structured in frames marked by alignment words it is also known to verify frame synchronization a posteriori by testing the transmission error rate detected by means of the block code, this error rate having to remain low (below a particular threshold) during each frame if the word regarded as the frame alignment word conforms to the specific position that the latter should have relative to the blocks defined by the code.
An object of the present invention is to accelerate the frame resynchronization process in the case of a synchronous digital bit stream divided into blocks by a block code and structured in frames, and consequently to reduce loss of information during frame resynchronization.